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ANDERSON COOPER, CNN ANCHOR: Here in Boston, two bombs, two brothers accused of turning the marathon into chaos and carnage.

Now we follow the trail of terror from the Boston suburbs to a war-torn part of Russia, investigating who the suspects were, how the deadly plan took place and crucially what turned a pair of striving immigrants into alleged killers.

DREW GRIFFIN, CNN INVESTIGATIVE CORRESPONDENT (voice-over): From the moment two bombs exploded at the finish line of the Boston Marathon...

UNIDENTIFIED MALE: Oh, my God. Oh, my God.

GRIFFIN: ... another race began to catch the perpetrators of the deadly attack.

RICHARD DESLAURIERS, FBI SPECIAL AGENT IN CHARGE: Today, we are enlisting the public's help. We are releasing photos of these two suspects

GRIFFIN: April 18, 5:21 p.m., these two men, later identified as 26-year-old Tamerlan Tsarnaev and his 19-year-old brother, Dzhokhar, become the most hunted fugitives in America, their pictures splashed across every TV, phone and computer in the country.

DESLAURIERS: Somebody out there knows these individuals as friends, neighbors, co-workers, or family members of the suspects.

GRIFFIN: Less than five hours after the images are released, the suspects resurface, with deadly consequences. At 10:20 p.m., an MIT police officer is found shot dead in his patrol car.

Just blocks away, around 11:00 p.m., a 26-year-old Chinese entrepreneur living in Boston is carjacked at gunpoint in his Mercedes SUV by two men claiming to be the Boston bombers.

ERIC MOSKOWITZ, "THE BOSTON GLOBE": Along the way, he hears them talk about Manhattan. They ask him about whether his car could go to New York.

GRIFFIN: The suspects stop at an ATM to withdraw $800 cash using their hostage's bank card and password. Back in the car, the victim's cell phone rings. It's his roommate asking where he is.

MOSKOWITZ: Tamerlan says, you know, pick up the phone. If you say anything in Chinese, I will kill you, because if he's speaking in Chinese, he may be calling for help.

GRIFFIN: En route to the expressway, they make a crucial pit stop.

MOSKOWITZ: Heading towards Interstate 95, they realize they need gas. They pull up to a gas station on the Charles River in Cambridge. The younger brother gets out with Danny's credit card to get gas. Moments later, he knocks on the window and says cash only. He then has to go into the Food Mart. That leaves just Danny and Tamerlan in the front two seats.

Tamerlan has, for some reason, the gun down in the pocket and is fiddling with the GPS. And Danny realizes, that's his time.

GRIFFIN (on camera): While one brother is inside paying for the gas, the carjacking victim bolts from the SUV, sprinting to the gas station across the street.

TAREQ AHMED, GAS STATION MANAGER (through translator): He fell. collapsed here. He fell right here and he appealed to me, please, please call the police. There are people out there who want to kill me, people who want to make sure I die. They have guns with them and they have a bomb with them.

GRIFFIN: In his rush to escape, the carjacking victim leaves his cell phone in the stolen SUV. Police trace its signal to Watertown, where Andrew Kitzenberg is hanging out in his living room, watching the hockey game.

ANDREW KITZENBERG, WITNESS: I heard pops outside. And so I ran to the window. And when I went to the window, I saw two shooters behind an SUV.

ED DEVEAU, WATERTOWN POLICE CHIEF: The two brothers stopped, get out of the vehicles and immediately started shooting at my officer.

GRIFFIN (on camera): Without provocation?

DEVEAU: Right. They took the gunfight to us.

GRIFFIN (voice-over): Kitzenberg runs upstairs to his bedroom, taking out his iPhone and starts taking photos of the surreal scene unfolding outside his window.

DEVEAU: There's a serious gunfight going on. The second person on the scene, one of my sergeants, he pulled up and he immediately gets at least one shot right through his windshield. And he decides to put the car in gear and lets it roll down the street while he's able to get out and take up a position so he's a little bit safer.

GRIFFIN (on camera): At some points, more than just guns, right? It's explosive devices.

DEVEAU: Right. One of them goes and pops the trunk of one of their vehicles and hurls something at our officers.

GRIFFIN: Did you see the bombs being thrown?

KITZENBERG: I did. Yes, I saw the explosives being thrown. And then I could see them reaching down into backpacks. I actually saw them take out the pressure cooker bomb and put it right at their feet.

GRIFFIN (voice-over): The brothers had another bomb just like the ones they're suspected of using at the marathon.

KITZENBERG: I actually saw a spark from the bomb and that's when I immediately hit the ground. I could feel it. I could feel it shake the house.

At that point, one of the shooters actually started charging the officers, running down the street, still firing his weapon.

DEVEAU: And they are, literally, about 10 feet away from each other exchanging gunfire. And then he runs out of ammunition, the bad guy. And one of my officers is able to tackle him and put him to the ground.

KITZENBERG: I looked back up and the other brother got back into the car and he had turned it around in the street and started accelerating up Laurel Street towards the police, towards the vehicles, basically flooring it.

DEVEAU: Somebody, at the last minute yelling get out of the way, and they dove out of the way as he came roaring through and ran over his brother.

GRIFFIN: As Tamerlan lies dying in the street, 19-year-old Dzhokhar Tsarnaev drives straight through a police barricade and escapes.

DEVEAU: The car that he abandoned a little further down the street, there was blood in that vehicle. So we knew he was wounded. We just didn't know how bad.

GRIFFIN: Wounded and on the loose. At dawn on Friday, an entire region is put on lockdown, as a convoy of law enforcement personnel and equipment rolls into Watertown.

GOV. DEVAL PATRICK (D), MASSACHUSETTS: We're asking people to shelter in place, in other words, to stay in doors with their doors locked.

GRIFFIN: During the day Friday, door-to-door, room-to-room searches turn up empty and by 6:00 p.m. bad news.

COL. TIMOTHY ALBEN, MASSACHUSETTS STATE POLICE SUPERINTENDENT: We do not have an apprehension of our suspect this afternoon.

GRIFFIN: The voluntary stay-at-home order is lifted.

PATRICK: We are asking the public to remain vigilant.

GRIFFIN: After a day inside, Watertown resident Dave Henneberry walks out to get some fresh air and notices something amiss with the cover of his boat. He took a closer look.

DAVE HENNEBERRY, WITNESS: I got three steps up the ladder and I was -- I rolled it up. And I can see now through the shrink wrap. I didn't expect to see anything. And I look in the boat over here and look on the floor, and I see blood.

GRIFFIN: What happens next is a blur.

HENNEBERRY: Well, I know I took three steps up the ladder. I don't remember stepping down off the ladder.

GRIFFIN: In an instant, special law enforcement units, including SWAT teams and helicopters with infrared cameras, swoop in. Shots are fired. Police are certain their man is inside Henneberry's boat named the Slip Away II.

Hostage negotiators attempt to talk Dzhokhar into surrendering.

UNIDENTIFIED MALE: Come out on your own terms. Come out with your hands up. We know you're bleeding. We know you're tried.

UNIDENTIFIED MALE: I believe they tried numerous flash bang grenades. They tried to gas him out of the boat. It just wasn't working.

GRIFFIN: A SWAT team approaches.

UNIDENTIFIED MALE: He was sitting on the edge of the boat with one leg hanging over the side.

GRIFFIN: They tell our Anderson Cooper about the tense standoff.

JEFF CAMPBELL, MBTA TRANSIT POLICE SWAT: So you could see one hand was clear of any weapons, but, each time he went back the other way, his hand went down inside the boat out of our view. And each time he did that, we had to assume he was reaching for either a weapon, firearm, or some type of explosive ignition device to try to draw us in and then take us out in a suicide-type manner.

We got close enough that at one point where both of his hands were up, we could see there were no weapons in him, no ignition devices. We broke away from the shield protective cover and we just rushed him.

We put hands on him, grabbed him, and pulled him off the boat down on to the ground and got him over to where the medics are and the federal agents who were taking him into custody.

GRIFFIN: Just before 9:00 p.m., it's over. A bloody, seriously wounded and unarmed Dzhokhar Tsarnaev is taken alive.

Coming up, Chris Lawrence on the trail of terror and what could have triggered the deadly bombings.

The other, Tamerlan Tsarnaev, was 26, married with a young daughter, an aspiring amateur boxer who had a dream.

LUIS VASQUEZ, KNEW SUSPECT: His goals in boxing, clearly, he wanted to be a world champ one day. And I think he could have achieved that.

LAWRENCE: Luis Vasquez met Tamerlan as a sophomore at the Cambridge Rindge and Latin School in 2004.

VASQUEZ: He could be intimidating because people knew he was a boxer and he had more of a reserved personality to him. But if you went up to him, he would talk to you right back and he was very friendly.

LAWRENCE: But were there any clues of the violence to come?

VASQUEZ: They never expressed any desires to harm people. They did respect life at one point. They were friendly.

LAWRENCE: After high school, Tamerlan attended Bunker Hill Community College studying accounting until 2008, when he left college, but he continued to pursue his dream.

Boxing trainer Eddie Bishop met Tamerlan at a tournament.

EDDIE BISHOP, BOXING TRAINER: He was a fighter that had a lot of skill and a lot of ability.

LAWRENCE: But Bishop questioned whether Tamerlan could ever become a boxing champ.

BISHOP: He had all the skill, but he lacked the hot, that fundamental ingredient to make you a champion.

LAWRENCE (on camera): The Tsarnaev family is from war-torn Chechnya and came here to the U.S. more than 10 years ago. Dzhokhar came first and then later his brother and two sisters joined the family.

(voice-over): Before coming to the United States, Tamerlan's father coached him in boxing. So he was already on his way to becoming an experienced fighter.

By 2009, Tamerlan was boxing in the Golden Gloves tournament in Salt Lake City, Utah. Fellow boxer Julian Pollard roomed with Tamerlan during the tournament. He remembers his swagger and sense of style.

JULIAN POLLARD, BOXER: He stood out and that was kind of, like, the perception that I got, flashy, confident, maybe cocky. I guess he backed it up in the ring. He could fight. He could punch. But he could put a guy out. When the time was right, if he had a big punch in him and the opening was there, he could finish a guy.

LAWRENCE: That ferocity apparently carried over outside the boxing ring. He was arrested that same year for slapping his girlfriend in the face during a fight over another woman. The following year, 2010, he married Katherine Russell, a Suffolk University student he met in a nightclub and had dated on and off for seven years. She converted to Islam.

Pollard saw him again that year at another boxing tournament in Lowell, Massachusetts. But he didn't seem the way.

POLLARD: In 2009, just a flashier guy, sharp dresser, kind of carried himself with a lot of confidence. And then the next year, he seemed like a humbler guy, dressed a lot more conservative, kind of, like, came to the fight as we all did, hoodie, jeans.

LAWRENCE: He also seemed more devoted to his Muslim faith.

POLLARD: He didn't talk that much about his faith to me until, like, the next year I saw him at the tournament. It was obvious that was a bigger part of his life. He felt like the immediate need to share it with me. He was really -- seemed like happy about where he was with his faith.

Z. TSARNAEV: I would like, momma, you to be covered, Islam is required -- is requiring for women to be covered. So I said it was really like not expected aspect. So I said give me some time, you know, Tamerlan.

LAWRENCE: Last year, Tamerlan traveled back to his homeland, a region of Russia racked by ethnic violence and Islamic extremism. When he returned to the United States six months later, he became more outspoken about his faith.

At the Islamic Society of Boston Cambridge mosque, Tamerlan stood up during a sermon and challenged the preacher in two separate public outbursts, one last November, another in January of this year.

ANWAR KAZMI, ISLAMIC SOCIETY OF BOSTON: So when the person who was giving the sermon began to talk about Dr. Martin Luther King, this man got up and objected. You know, he raised his voice, which is against the etiquette of the sermon. And people objected to it. And some people might have even asked him to leave the mosque if he didn't, you know, like what he was hearing.

LAWRENCE: But there were no indications that Tamerlan or younger brother Dzhokhar could be dangerous.

VASQUEZ: They cared about you. They cared about people around them. They cared for other people making great choices and creating opportunities for themselves, like they were trying to do for each other and themselves also. It just doesn't make sense.

LAWRENCE: Tamerlan's high school friend Luis Vasquez later coached Dzhokhar in soccer at the Cambridge Rindge and Latin School.

VASQUEZ: These guys had two traits. One was the leader of his family and really cared for them and the other one was just a social leader amongst his friends and on teams. He was co-captain of the wrestling team, the little brother.

LAWRENCE: Dzhokhar got a $2,500 academic scholarship and enrolled at the University of Massachusetts, Dartmouth. On September 11, 2012, an ironic date in retrospect, he took the oath to become a U.S. citizen.

Zach Bettencourt went to college with Dzhokhar.

ZACH BETTENCOURT, STUDENT: He completely acted American. I wouldn't know. I didn't even know he was from Chechnya.

JAY HONGLA, STUDENT: He was a very quiet, cool kid, very smart, extremely smart, classmates always asking for help. He helped all the time. He wasn't a troublemaker.

BRANDON ALEGI, STUDENT: I thought he was, like, really like -- he was a good kid, honestly. He seemed like a good kid. I used to like -- every time I saw him, I thought he was a nice person.

LAWRENCE (on camera): Those who thought they knew the brothers from high school, college, through boxing now realize they never really knew them at all.

(voice-over): The two brothers outwardly so different, on April 15, are accused of acting together to bomb the Boston Marathon.

Incredibly, just days after the bombings, Dzhokhar was spotted back on campus. Bettencourt saw him at the gym.

BETTENCOURT: We talked about the bombing for, like, five minutes. He seemed very nonchalant. He didn't like nervous or anything. He seemed a little maybe tired and off, because I asked him how he was doing.

He was like, yes, I haven't been doing much lately, so I decided to come to the gym. I'm just like, well, whatever.

LAWRENCE: Bettencourt says his reaction wasn't surprising at the time.

Z. TSARNAEV: I was just talking about how, yes, in Iraq and Afghanistan, these things happen all the time. And here it's crazy how this would happen. And he just said -- he was like, yes, tragedies happen. And these things happen around the world. Like, it's crazy.

LAWRENCE: For Julian Pollard, he hadn't thought much of his former boxing buddy until last week. That's when he remembered that he had kept Tamerlan's number.

POLLARD: I forgot it was in the phone until I saw his picture on the news. So then I hit the T. and, sure enough, his name was right there. And I was like, wow, it's definitely this dude.

LAWRENCE: But were there other clues in his past?

Coming up, the Russian connection. Nick Paton Walsh investigates what Tamerlan was doing on his trip back to his homeland.

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NICK PATON WALSH, CNN CORRESPONDENT (voice-over): The North Caucasus, a region ravaged by years of violence and bloodshed. The Tsarnaev family is originally from here, but they left for a better life in the United States.

Tamerlan Tsarnaev returned to this part of the world last year for a six-months visit that is now raising a lot of questions.

TOM FUENTES, CNN CONTRIBUTOR: Who did he meet with over there? What did he learn? What did he do? Did he become part of a cell? Did he get training on how to assemble explosives? Did he get money? Did he receive encouragement, funding, direct support to come back to the U.S. and attack? And those are things that I don't think we will ever know.

WALSH: What we do know is that in January of 2012, Tamerlan left the United States on a flight bound for Moscow. He made his way to Dagestan, a Russian republic next to his family's ancestral homeland, Chechnya, today also a part of Russia.

PATEIMAT SULEIMANOVA, AUNT OF SUSPECTS (through translator): He could radiate light and warmth.

WALSH: His extended family in Dagestan welcomed him warmly.

SULEIMANOVA (through translator): He smiled a lot. And I asked him, is this your customary American smile? He was more of an American.

WALSH (on camera): Much of what Tamerlan Tsarnaev did here remains a mystery. We know that he stayed for some weeks here at his parents' home, keeping mostly to family and friends, helping his father out in his businesses around town and often sleeping in.

(voice-over): The Tsarnaev family is part of a Chechen community here, like many other Chechens, displaced from their homeland. After the Second World War, tens of thousands of Chechens perished in a mass deportation ordered by Joseph Stalin. In the 1990s, Chechens fought and won a bloody war for independence from Russia.

It was during this violence that the younger Tsarnaev brother, Dzhokhar, was born and given the name of a Chechen leader.

THOMAS DE WAAL, CARNEGIE ENDOWMENT: It's a sure sign of Chechen patriotism in the family that in 1993, they called that little son Dzhokhar, after Dzhokhar Dudayev, who was the pro-independence president.

WALSH: But Chechen independence wouldn't last. Another war with Russia would kill thousands and Chechen extremists would wage a campaign of terror across Russia, killing 186 children at a school in Beslan and more than 100 theatergoers in Moscow.

The Tsarnaev family escaped the violence of the region by making their way to the U.S.

VASQUEZ: He would tell that he is from Chechnya.

WALSH: Luis Vasquez was friends with Tamerlan in high school.

VASQUEZ: That's where he's from. That's where he told me he had struggles. He didn't really elaborate.

DE WAAL: There's a whole generation of Chechens who grew up with their families being displaced, relatives being killed and so on.

WALSH: Thomas de Waal has studied and written about the region.

DE WAAL: For most people, that's just a traumatic experience, but obviously for a small minority, this is something that is in their DNA that drives them.

WALSH: And like many displaced Chechens, Tamerlan may have struggled to fit in. Chechnya is still home to militant separatist groups, and the home Tamerlan's father grew up in has been destroyed by war. But relatives still live hire. Tamerlan traveled here during his trip last year.

ZAINALBEK TSARNAEV, GREAT UNCLE TO SUSPECTS (through translator): came to see me. We talked. I said, come here, guy. Are you studying, I asked. I'm studying, he said. There was nothing criminal about him.

WALSH: De Waal says this region has also seen a growth in Islamic extremism, becoming more anti-Western and anti-American.

DE WAAL: If you're a young jihadist from this region, you certainly blame Russia as the kind of evil empire that attacked and oppressed you, and also the West basically did nothing to intervene when Russia was bombing Chechnya.

WALSH: It's not known if Tamerlan met with any extremists during his visit to Chechnya.

On his YouTube page in a play list called "Terrorism," there was a link to this video of a small-time militant, Abu Dujan. Abu Dujan and Tamerlan were both in Dagestan last year.

According to Dagestani police, Abu Dujan's group ran a training camp in the woods and made these videos demonstrating how to mix and prepare homemade explosives or use a cell phone as a detonator. And, says the local police chief, the militants trained foreigners.

(on camera): What did the foreigners learn in the woods?

ASKHABALI SAURBEKOV, KIZILYURT POLICE CHIEF (through translator): I can't talk about the number of foreigners, but they met to exchange their bandit experience.

WALSH (voice-over): He says the militants trained Chechen men who live in other countries.

(on camera): There are reports that Dujan was observed at the mosque and he was observed meeting Tsarnaev. Do you know this?

WALSH: Did Abu Dujan and Tamerlan meet? We don't know. This past December, Abu Dujan and other militants were killed when Russian special forces hit their hideout.

FUENTES: Maybe he's a hero of the brother; maybe he isn't. But Tamerlan watches these videos, and then this individual gets killed. Now, logic would tell you, well, who would you be mad at? Runners in the Boston Marathon? What did they have to do with it? Would you be mad at the United States? What did the U.S. have to do with it?

PATON-WALSH: No one will ever know. But we do know that on his last visit to Dagestan, Tamerlan's family noticed a change in him.

PATEIMAT SULEIMANOVA, AUNT TO TSARNAEV BROTHERS (through translator): We were happy that he didn't become a drug addict. He didn't become an alcoholic. He was on the path of Islam.

PATON-WALSH (on camera): His family says that during his time here, Tamerlan would often visit the central mosque behind me for Friday prayers as well as other mosques across this town. Telling his family how central Islam was to his life and that it was a religion of peace.

(voice-over): That's why he doesn't believe that Tamerlan and Dzhokhar could have bombed the Boston Marathon. SULEIMANOVA (through translator): In Islam, killing a non-Muslim is like killing all of humanity. And killing a Muslim is like killing the whole world.

PATON-WALSH: Last year, Tsarnaev's parents moved back to Dagestan from the U.S. Their aunt says the mother was in shock when she saw her son's faces splashed across television screens here.

SULEIMANOVA (through translator): She grabbed the television screen and started screaming, shouting, "It can't be. I don't believe it. It can't be."

PATON-WALSH: Their mother still cannot believe her sons may be responsible.

ZUBEIDAT TSARNAEV, MOTHER OF TSARNAEV BROTHER: I really feel sorry for all of them. Really feel sorry for all of them. But I do not want to believe that these were my sons, OK. And I don't believe. I don't believe.

PATON-WALSH (on camera): The people investigating Boston, they say that Tamerlan's faith drove him, what they say he did, in Boston.

Z. TSARNAEV: I don't think so. I don't think so. I don't think that Tamerlan did it. I don't think that faith would bring him into it.

PATON-WALSH (voice-over): On Thursday, the parents held a press conference. They are helping with the investigation. Both were interviewed earlier by U.S. authorities.

ANZOR TSARNAEV, FATHER OF TSARNAEV BROTHERS (through translator): they asked about our children. How did they live? What did they do? What were their interests?

Why would the Tsarnaev brothers do it? How did they do? And, most importantly, who might have helped them?

JULIETTE KAYYAM, FORMER ASSISTANT SECRETARY OF HOMELAND SECURITY: What they want to do is ensure that there are no other ties to any other groups that might try to follow up with this or plan something.

GRIFFIN: Juliette Kayyam is a former assistant secretary of homeland security.

KAYYAM: The other piece is building a strong case and making sure that case doesn't fall apart. That's why the indictment released this week was very, very careful to say, look, we know he used weapons of mass destruction. We know he killed people. And that's enough to give him the death penalty.

GRIFFIN: To build the case, and find the answers, hundreds of federal investigators are now working around the clock, digging through debris, analyzing bits of bombs in one of the nation's largest terrorist investigations since 9/11. Neighborhoods like this one across Boston are being visited by the FBI.

This man says five FBI agents showed up at his door to question his son about tweets sent to the younger brother.

And on the college campus where Dzhokhar went to school, friends there tell CNN that they, too, have been visited by the FBI.

ERIN BURNETT, CNN ANCHOR: "OUTFRONT" next, breaking news.

WOLF BLITZER, CNN ANCHOR: We'll hear about the terrifying line through Boston.

ANDERSON COOPER, CNN ANCHOR: There is a lot of news.

GRIFFIN: Leads are still pouring in, and new details emerging daily.

UNIDENTIFIED MALE: Breaking news on the Boston bombing.

COOPER: Her attorneys say she knew nothing about what her husband was allegedly planning.

BLITZER: A Massachusetts gas station manager is now speaking out.

GRIFFIN: Among the leads, details on the bombs used by the Tsarnaev brothers. Bombs that started with ordinary pressure cookers bought at a Boston department store. While deadly, the bombs were simple and cheap. Packed with nails and ball bearings to cause maximum carnage.

FUENTES: All of the equipment, all of the supplies involved in the Boston bombings was probably under a hundred dollars.

GRIFFIN: And where would two brothers in Boston learn how to take a pressure cooker like this and make it into a bomb like that? Right here, of course. It's on the Internet.

PAUL CRUICKSHANK, CNN TERRORISM ANALYST: The striking similarity between the Boston devices and a recipe which was put out by "Inspire" magazine in the summer of 2010. "How to Build a Bomb in Your Mom's Kitchen."

GRIFFIN: Paul Cruickshank is a terrorism analyst for CNN.

CRUICKSHANK: Now, these similarities include the fact that pressure cookers were used; an explosive shrapnel was used; a low- grade explosive powder was used. But, also, very specific stuff like the fact that "Inspire" magazine told followers that they should glue shrapnel inside the pressure cookers. Now, that's what was done in Boston.

GRIFFIN: Published by al Qaeda in the Arabian Peninsula in English, the magazine was started by Anwar al-Awlaki, the American- Yemeni cleric who was killed by a U.S. drone strike.

Cruickshank has been tracking al-Qaeda and this magazine for years. He says investigators are looking closely at whether the brothers Tsarnaev got their bomb recipe here. That could have larger implications.

CRUICKSHANK: One of the largest concerns of Western counterterrorism officials is that "Inspire" magazine is still publishing new issues. In an issue that was put out recently, the magazine stated, "We are publishing America's worst nightmare."

GRIFFIN: As investigators sift through evidence, the bits of bombs blown across the crime scene should yield other clues, too.

RAYMOND LOPEZ, FORMER FBI SUPERVISORY SPECIAL AGENT: What were the components used in creating these devices? Coupled with that, you're going to have an investigation or laboratory sciences working on things like fingerprints, hair and fibers that may be included in there. And also tool markings.

GRIFFIN: Ray Lopez is a former FBI explosives expert.

LOPEZ: These things were made with tools, so there's going to be a look for tool marks that are left after these things were done. And that's going to be matched to any tools that they could find at any of the search sites that the investigators are working at.

GRIFFIN: Officials now say at least one of the bombs was detonated with a remote control device similar to those used to control toy cars.

Federal investigators must also now ask themselves hard questions like did they drop the ball with Tamerlan Tsarnaev? Two times, the Russian government approached the U.S. with concerns about the older brother, who later traveled to Chechnya.

Upon his return to the U.S., Tamerlan posted this video, showing clear extremist tendencies.

SEN. DIANNE FEINSTEIN (D), CALIFORNIA: What did he do when he went back for six months? Did he sit in his aunt and uncle's home for six months? Or was he doing something else? And when he came back to this country, why didn't it ring a bell with the FBI intelligence unit that he should be checked out and vetted again?

GRIFFIN: The FBI has told officials the bureau looked into the older brother in 2011 and found no red flags. The CIA was also approached by the Russians with similar concerns about Tamerlan. Questions were also raised by officials about the Tsarnaev's mother. Both she and the older brothers' names were added to a terrorist identities database. But the investigation appears to have gone nowhere.

Was Tamerlan Tsarnaev radicalized overseas? Maybe so. But his uncle says Tamerlan's views more likely came from someone in the U.S.

RUSLAN TSARNI, UNCLE OF TSARNAEV BROTHERS: I heard that talking from Tamerlan, where that might be coming from. And then he says, "Oh, yes, there is -- yes, there is such a thing. There's a person, sort of some new convert into Islam."

GRIFFIN: That convert, a Muslim extremist right here in the Boston area.

TSARNI: He said this person just took his brain, just brainwashed him completely. Tamerlan is off now. There's no -- any obedience in respect to his own father.

GRIFFIN: The uncle says Tamerlan told him he quit listening to music and got very conservative after talking to the extremist, whose name was Misha. Tamerlan's mother says Misha was a good influence on her son.

Z. TSARNAEV: When Misha visited us, we just kind of -- he just opened our eyes, you know, really wide about Islam. He was really -- he was devoted, and he's a very good, very nice man.

GRIFFIN: Dzhokhar Tsarnaev, the surviving brother, told investigators that they conceived the attack. And that they were self-radicalized jihadists, inspired in part by the U.S. wars in Afghanistan and Iraq.

Late this week, New York officials had their own news about the bombers.

MAYOR MICHAEL BLOOMBERG, NEW YORK CITY: Last night, we were informed by the FBI that the surviving attacker revealed that New York City was next on their list of targets.

GRIFFIN: The officials said Dzhokhar Tsarnaev indicated the brothers intended to take their remaining explosives and blow them up in Times Square.

Should investigators believe the brothers plotted and carried out the act on their own? Still, more questions. Few clear answers.

We may never know who Tamerlan met with overseas. Or what happened to him. But we do know this. He came back a changed man.

JOHN KERRY, SECRETARY OF STATE: We just had a young person who went to Russia and Chechnya who blew people up in Boston. So he didn't stay where he went, but he learned something where he went and he came back with a willingness to kill people.

GRIFFIN: Coming up...

COOPER: How close were you to the second explosion?

ADRIANNE HASLET-DAVIS, BOMBING SURVIVOR: I was right in front of it.

GRIFFIN: ... Anderson Cooper finding Boston's strong.

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UNIDENTIFIED MALE: Do you have communication with your guy? Go ahead.

COOPER: The Boston Police Department has just tweeted "suspect in custody."

(voice-over): Five days after the blast, the manhunt was over. People of Boston seemed to breathe a collective sigh of relief. Ready to get back to business. Ready to live life again.

UNIDENTIFIED MALE: It's time for our ceremonial first pitch.

COOPER: And for Boston, that means baseball. It was a time of celebration amidst the sadness. Triumph over tragedy. Or, as they say here, not just strong, Boston Strong.

EDWARD DAVIS, BOSTON POLICE CHIEF: There's not anybody that's lived in Boston for any period of time that wasn't touched by, somehow, the victims down there.

COOPER: Boston police chief Edward Davis.

DAVIS: So we were all in it together. And that's what Boston Strong meant. We were all going to stick this out and work together.

COOPER: Work together to save lives.

To catch the suspects.

UNIDENTIFIED FEMALE: They have the suspect. They know exactly where he is.

Today, the streets of Copley Square are, once again, open. And the injured are healing from the blast that changed their lives forever.

HASLET-DAVIS: We're lucky to be alive.

COOPER: Adrianne Haslet-Davis was watching the marathon with her husband, Air Force Captain Adam Davis, who served in Afghanistan.

(on camera): How close were you to the second explosion? HASLET-DAVIS: I was right in front of it. Right in front of the -- the business where it was. So I felt the direct impact.

COOPER: When the first explosion went off, what did you think?

HASLET-DAVIS: There was this silence. And I thought -- I clung to Adam, my husband. And I thought, you know, there's going to be another explosion. And as soon as I thought that, I started -- I started saying, "Oh no, oh no, oh, no." You just have no idea where it's going to come from.

And then, all of a sudden, I was blown in the air and landed almost in a pretzel with him.

COOPER (voice-over): The blast was so powerful they say they were thrown about five feet.

HASLET-DAVIS: And I said, "I think we're OK." And I couldn't believe it. I couldn't believe that we survived and that we weren't hurt at all. And I didn't feel any pain. I had no idea what had happened.

And then I sat up and I tried to -- he said, "We've got to get out of here" or something like that. And I sat up and we tried to move.

And I said, "Oh my gosh, my foot. There's something wrong with my foot." And he lifted up my leg, and we just lost it and just started screaming bloody murder. It was really bad.

COOPER (on camera): What did you do then? When you realized what had happened to your foot?

HASLET-DAVIS: I just went into survival mode. I went into "I've got to do something about this. I can't lose my foot."

COOPER (voice-over): She couldn't lose her foot because Adrianne is a dance instructor. It's her passion. Her life. And even as she was wheeled into the operating room, Adrianne remained optimistic. When she woke, she remembers feeling her toes.

(on camera): When did you realize you didn't have a foot?

HASLET-DAVIS: I was -- I woke up and my parents were there. And I hugged them and kissed them and I said, "Mom, can you help me? I feel like my foot is falling asleep, because it feels like my ankle is -- is falling off of the pillow and my foot is half on." And I realize now that that was phantom pain, because she looked at me and said, "Adrianne, you don't have a foot. Your foot is gone." And I just lost it. It was really hard to hear.

COOPER: You're determined to dance again.

HASLET-DAVIS: I am.

COOPER: Dancing is really important to you? HASLET-DAVIS: It is so important to me. It's my life.

COOPER: You're incredibly optimistic.

HASLET-DAVIS: I try to stay on the positive side. I can either stay in bed and cry and be really upset. And I do have moments of that. But -- or I can say I'm going to, you know, run the marathon next year and conquer and be good.

I would never let one of my students come in and say, "Oh well, I can't do this anymore because I hurt my arm or I lost my leg" or if they came in with a prosthetic, I would say, "I'll teach you. That's fine. We're going to make that work." And I would be (UNINTELLIGIBLE) if I didn't spend that time with them.

COOPER: And you want to run the race next year?

HASLET-DAVIS: I do. I can't believe I said that.

I'm only 32. I don't want this to be the end. So whether it's running the marathon or walking the marathon or crawling the marathon and being the last one across, I'm OK with that. I didn't say I'd win it. But I am defiant. I want to -- I want to come out stronger.

COOPER: Stronger. Boston stronger. Just like the city where first pitch signals a new start, like the city that's rebuilding, rebounding, like the city that terror couldn't stop.

UNIDENTIFIED MALE: We are one Boston. No adversity. No challenge. Nothing can tear down the resilience in the heart of a city and its people.

(COMMERCIAL BREAK)

COOPER: Today, Boston is back in business. And Adrianne, the dancer we introduced you to, is getting better. I plan to keep in touch with her during her recovery. She'll have some challenges ahead, no doubt about it. Not the least of which, making good on her promise to teach me how to dance.